Syozo Imai

Surface Conductance of Metal Surfaces in Air Studied with a Force Microscope

With a force microscope, the surface conductances of various metals in air were investigated by measuring both the contact force and the current between the metal surface and the W lever simultaneously. When the Ag surface approached the W lever, current began to flow only in the strong repulsive-force range in contrast to the Au surface, so that the Ag surface seemed to be oxidized slightly. In the case of the Ti surface, current was not observed even in the strong repulsive-force range, so that the Ti surface seemed to be oxidized thick enough to sustain this strong repulsive force.

Simultaneous Observation of Atomically Resolved AFM/STM Images of a Graphite Surface

We constructed an AFM/STM system with a conductive lever. We applied this system to the observation of a graphite surface in air. As a result, for the first time, atomically resolved AFM and STM images were obtained simultaneously, and it was found that the lattice pattern appearing in the AFM image was different from that in the STM image. In both AFM and STM images, distortion of lattice periodicity due to frictional effect was observed.

Thermoanalytical Studies on Preparation Conditions of Superconducting YBa2Cu3O7-y

We studied the decarbonation process of a ternary mixture of Y2O3, BaCO3 and CuO or Cu2O by TG-DTA analysis to clarify the optimum condition for the preparation of YBa2Cu3O7-y ceramics, which are superconducting above 90 K. We found that a narrow temperature region, 940~950°C, is desirable for the decarbonation process. Furthermore, we observed an anomaly at ~600°C in DTA curves of stoichiometric YBa2Cu3O7-y ceramics and nonstoichiometric Y–Ba–Cu–O samples. From the strong correlation between the magnitude of this anomaly and the composition of the ceramics, the anomaly was attributed to the orthorhombic-tetragonal phase transition in the YBa2Cu3O7-y phase. It was also found that the composition dependence of Tc of the ceramics correlates strongly with that of the magnitude of the anomaly.

Submillimeter wave response of tunnel junctions with an insulating barrier containing magnetic impurities

We have investigated PAT (Photon-Assisted-Tunneling) effect induced by SMMW (Submillimeter-Wave) in SIS point-contacts with an insulating barrier containing magnetic impurities: a magnetic barrier. The junctions turned out to have severely suppressed dc and ac Josephson currents, which makes them promising junctions for a high frequency mixer using PAT effect.

Damping Factor of the Riedel Peak in Bridge-Type Josephoson Junctions

The Riedel peak in the Josephson current in bridge-type point-contact Josephson junctions has been studied by measuring the frequency dependence of the maximum height of the fundamental ( n =1) Shapiro-step in the energy range near \(\hbar\omega{\sim}4\varDelta_{0}\) with a submillimeter-wave laser, where Δ 0 is the real part of the energy gap. The imaginary part of the energy gap, Δ 2 , has been estimated from the observed rounding of the Riedel singularity. As a result, the damping factor \(\delta{=}\varDelta_{2}/\varDelta_{0}{\gtrsim}0.05\) has been obtained. The value of δ∼0.05 seems to be the lowest value which is intrinsic to bridge-type junctions. It has been also found that the value of δ is large in the junctions which show strong heating effects.

On the Reflection and Transmission Coefficients Associated with a Lowest Exciton Level (in the Case of Weak Coupling between Polariton and Phonon)

With use of the photon Greens function, the reflection and transmission coefficient associated with a lowest exciton level based on the “polariton” picture was calculated. To consider the effects of the spatial dispersion due to the translational motion of excitons, the “specular” and “diffuse” boundary conditions are adopted. The calculation based on a simple model for polaritons shows that the minimum of the transmission coefficient arises in the neighbourhood of the transverse exciton energy, and the hump may be expected at the longitudinal exciton energy.

Nonlinear response of tunnel-type Josephson junctions at submillimeter-wave frequencies

We have fabricated tunnel-type Josephson junctions with very small junction area by pressing the tip of a sharpened Nb wire onto a thinly oxidized Sn film, and studied their submillimeter-wave responses. It turns out that the junctions have a very high cutoff frequency, and are suitable for the investigation of the response to the submillimeter waves with the frequency higher than 1 THz. From the frequency and laser-power dependence of the Shapiro step, it is found that the tunnel-type junction shows much larger higher-order steps, and accordingly has much stronger nonlinearity than the bridge-type junction in the submillimeter-wave region. The large enhancement of the steps due to the Riedel singularity is also observed in the tunnel-type junction. In addition to the Shapiro steps, photon-assisted tunneling steps are observed clearly, reflecting the large and sharp gap structure in the dc IV curves of the tunnel-type junctions.

Frequency Dependence of the Submillimeter-Wave Response in the Tunnel-Type and Bridge-Type Josephson Junctions

We have fabricated ultrasmall tunnel-type Josephson junctions with Nb–SnOx–Sn film point-contacts. Using these junctions, we have measured for the first time the Shapiro steps in the submillimeter-wave region and have compared the strength of steps of this junction with that of the bridge-type Nb–Sn ingot point-contacts. We found that the tunnel-type junction shows a sharper Riedel peak than the bridge-type junction.

Correlation between Types of Junction and Submillimeter-Wave Responses in Point-Contact Josephson Junctions

It has been found that point-contact Josephson junctions can be clearly classified into bridge-type and tunnel-type by introducing new parameters characterizing their I-V curves. Nearly-ideal tunnel-type junctions with Nb–SuOx–Sn (Film) point-contacts were obtained. Tunnel-type junctions showed larger gap energies than bridge-type junctions in Nb–SnOx–Sn and Nb–NbOx–NbN point-contacts. The submillimeter-wave responses were measured in Nb–Nb point-contacts, and it was found that the contacts with the best performance were bridge-type junctions with a sharp gap structure; tunnel-type junctions showed rather bad performance. On irradiation with laser light, the center of the Shapiro step moved to the lower-current side in bridge-type junctions but to the higher-current side in tunnel-type junctions. These differences can be explained by differences in the heating effects between bridge-type and tunnel-type junctions.

YBa2Cu3O7-z/Pr0.5Ca0.5MnO3-y/YBa2Cu3O7-z Sandwich Type Josephson Junctions

YBa2Cu3O7-z/Pr0.5Ca0.5MnO3-y/YBa2Cu3O7-z sandwich type junctions are fabricated using pulsed laser deposition and photolithography processes. Clear Shapiro steps are observed under microwave irradiation. Along with the magnetic field response, the results prove the existence of a Josephson effect at this junction. A superconductor-insulator-superconductor (S-I-S)-like transfer process is suggested from the temperature dependence of Ic and Rn.